A synchronous motor is designed to maintain synchronism with the AC power supply by running at a constant speed that is directly proportional to the frequency of the AC voltage applied to it. This synchronization is crucial for the motor to operate efficiently and perform its intended tasks.
Here's how a synchronous motor maintains synchronism with the AC power supply:
Operating Principle: The synchronous motor's rotor has a series of electromagnets or permanent magnets that create a magnetic field. When the AC voltage is applied to the motor's stator windings, it generates a rotating magnetic field. The rotor's magnetic field interacts with the stator's rotating magnetic field, causing the rotor to turn.
Synchronization with Frequency: The key to maintaining synchronism is that the motor's speed is determined by the frequency of the AC voltage applied to it. The synchronous motor's speed, called the synchronous speed (Ns), is given by the formula:
Ns = (120 * f) / P,
where:
Ns is the synchronous speed in revolutions per minute (RPM),
f is the frequency of the AC power supply in hertz (Hz), and
P is the number of poles in the motor.
Matching Poles and Frequency: To maintain synchronism, the motor's rotor must have the same number of poles as the stator's rotating magnetic field and the applied frequency. For example, if a motor has four poles and is connected to a 60 Hz power supply, the synchronous speed would be 1800 RPM. The motor would need to run at this speed to stay synchronized with the power supply's frequency.
Slip: Unlike asynchronous (induction) motors, synchronous motors don't experience significant slip. Slip is the difference between the actual rotor speed and the synchronous speed. In a synchronous motor, slip is nearly zero because the rotor is always trying to keep up with the rotating magnetic field.
External Load and Excitation Control: If the motor experiences a sudden increase in load, its speed may drop slightly due to increased mechanical resistance. To counteract this and maintain synchronism, synchronous motors are equipped with a separate DC power supply for field excitation. By adjusting the excitation current, the strength of the rotor's magnetic field can be controlled, enabling the motor to counteract changes in load and maintain its speed.
In summary, a synchronous motor maintains synchronism with the AC power supply by operating at a speed that corresponds directly to the frequency of the AC voltage applied to it. This is achieved through careful design of the motor's rotor and stator configuration, as well as control over field excitation.